Process for manufacturing high-yield, high-strength pulp at low energy

ABSTRACT

A low energy process for the manufacture of high yield pulp that involves the processing of chemically-treated chips or wood fiber at high stresses or intensity.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is related to U.S. Ser. No. 60/066,259 filed Nov. 20, 1997.

BACKGROUND OF THE INVENTION

[0002] i) Field of the Invention

[0003] This invention relates to a process for manufacture of high yield pulp, more especially a low energy process.

[0004] ii) Description of Prior Art

[0005] Although refining of wood chips into pulp has been applied commercially since the 1960's, the mechanism of refining is not completely understood. A major step in this understanding was made by Miles and May in their articles in the Journal of Pulp and Paper Science 16(2):J63 (1990) and Paperi ja Puu 73(9):852 (1991). They developed a set of equations based on a mass and energy balance to calculate the consistency and velocity of pulp within a refiner. From this approach the residence time and specific energy per bar impact or refining intensity can be determined. Experimental and empirical relationships can be developed between pulp properties and specific energy and refining intensity. However, because it is not known how the wood fibres react to the stresses imposed in refining, it is not possible to make these relationships analytical or predictive.

[0006] Earlier work on untreated chips in RMP and TMP refining suggested that an energy savings of approximately 15% to 20% was possible by increasing refining intensity. (See Tappi Journal 74(3):221 (1991), Journal of Pulp and Paper Science 19(1):J12 (1993) and U.S. Pat. No. 5,167,373). The higher intensity treatment was implemented by either increasing the disc rotational speed from 1200 to 1800 RPM or reducing the inlet consistency to the refiner from 20 to 10%. This approach was most effective when the first refining stage was operated at high intensity and the second stage was operated with conventional conditions. The optimum energy saving which maintained pulp and fibre properties was obtained by putting a smaller portion of the total specific energy in the first, high intensity stage. A typical split in specific energy between the first and second stages of refining would be 40/60. Increasing the refining intensity in the first stage or the proportion of the specific energy applied in the first stage further would lower the total energy to reach a given freeness. It would also lower the average fibre length and pulp strength, limiting the potential for additional energy savings.

[0007] The RTS Process, developed by Andritz, claims a reduction of at least 10% in specific energy over conventional TMP (U.S. Pat. No. 5,776,305). In this process, the chips are pre-treated above their glass transition temperature for 5 to 30 seconds before refining at higher than conventional intensities by means of higher disc speeds. Further pulp and fibre development takes place in a second refiner operating under conventional conditions. Pulp strength and optical properties are the same or better than pulp produced by a conventional TMP Process.

[0008] A mild bisulphite pretreatment of chips at pH, 4.2 combined with high intensity refining gives an energy savings of 33% over conventional TMP as shown by Stationwala in Tappi Journal 77(2):113(1994). The energy savings by chemical treatment with sodium sulphite, 0.20 to 0.45% SO₃ content on wood, and refining, are additive.

[0009] Broderick et al. (U.S. Pat. No. 5,540,392) have shown that it is possible to reduce energy by up to 18% in a two-stage refining system. At least 65% of the total energy is applied in a low intensity first stage refiner. The remaining energy is applied in a high intensity second stage refiner. The pulp properties are reported to be at least as good as or better than that produced by conventional refiners. Using high intensity refining in the second stage seems to be in direct contrast to Paprican's approach. However, in the final analysis both strategies appear to be limited to energy savings in the order of 15 to 18%. Greater energy savings would lead to an unacceptable deterioration in pulp properties.

[0010] There are, however, certain chip pre-treatment processes which produce substantial changes in the properties of the raw material. Under these conditions high intensity refining results in greater energy savings without unacceptable compromises in pulp quality.

SUMMARY OF THE INVENTION

[0011] It is an object of this invention to provide a process for the manufacture of high yield pulp, especially a low energy process.

[0012] In accordance with the invention, there is provided a process for producing a high yield pulp comprising:

[0013] chemically treating a pulp precursor selected from wood chips or wood fibre to develop wood fibre of said precursor while maintaining fibre integrity, and

[0014] refining the chemically treated pulp precursor at a high refining intensity.

DETAILED DESCRIPTION OF THE INVENTION

[0015] A low energy process has been discovered for the manufacture of high yield pulp which comprises treating wood chips or wood fibre with sodium sulphite or other chemicals then processing the material mechanically using higher stress levels than are currently used in commercial applications.

[0016] In the case of treatment with sulphite, the sulphonate content on the chips is determined as %, by weight SO₃ is between 0.1 and 3%, more especially 1 to 3% and preferably 1 to 2.5%, by weight, on an O.D. (over dried) basis.

[0017] The compressive forces in this process are in the range of 0.1 to 30 MPa more especially 0.5 to 30 mPa which is several orders of magnitude greater than that in conventional refining. This greater compressive force can be applied in a variety of devices including rolling mills, vibratory inertial refiners, and refiners that have been modified to deliver higher stresses or intensities than is conventional. Up to 70% less energy is consumed in this process giving the same pulp and fibre properties as conventional refining.

[0018] The materials or precursor used in this pulping process can be either softwoods or hardwoods in the form of chips, fibres or pulp. This material is impregnated with sodium sulphite or other chemicals that soften the chips and develop the wood fibres. This material is then processed into pulp by mechanical action in devices that deliver higher stresses or intensities than are now used conventionally. The key to this discovery is that wood fibres can be separated and developed at very high stresses without being broken if they are sufficiently softened. The advantage of operating at these high levels of stress is that the pulp develops with fewer impacts which significantly lowers the energy requirement.

[0019] High refining intensities in a range of 200 to 2,000, preferably 250 to 1,000 and more preferably 300 to 700 J/kg per impact are applied to wood chips, fibres or pulp at stresses of the order of 0.1 to 30 MPa and at peak strain rates ranging from 50 to 1000 s⁻¹.

[0020] In contrast with the high refining intensities of the invention, conventional disc refiners operate at a refining intensity of 150 to 200 J/kg/impact.

[0021] This can be done in a vibratory inertial refiner (VIR) which consists of a conical rotor rolling on the surface of a conical stator. The force exerted by the rotor is concentrated along the line of contact with the stator which gives a higher compressive stress on the material than a conventional disc refiner, and which leads to higher levels of specific energy per impact.

[0022] Similarly, a rolling mill consisting of a series of rollers rotating within a cylindrical surface which exert line loads on the material that are in excess of compressive stresses exerted in conventional disc refiners and which lead to higher levels of specific energy per impact may be employed. This process can also be implemented with a disc refiner by increasing refining intensity by either increasing the rotational speed of the disc(s) or lowering the outlet consistency.

[0023] These levels of refining intensity compare with 150 J/kg per impact for a double-disc operating at 1200 RPM and an outlet consistency of 25%. This process can also be implemented by increasing refining intensity in a disc refiner by either increasing the rotational speed of the disc(s) or by lowering the outlet consistency. For instance, for a disc speed of 1800 RPM and an outlet consistency of 25%, the refining intensity was 330 J/kg per impact. Lowering the outlet consistency to 12.5% and maintaining the disc rotational speed at 1800 RPM, increased the refining intensity to 660 J/kg per impact.

[0024] Chemicals which may be employed to treat the pulp precursor prior to the high intensity refining include alkali metal sulphite or bisulphite, alkali metal hydroxide and alkali metal carbonate, especially sodium sulphite, sodium bisulphite, sodium hydroxide and sodium carbonate. The alkali metal hydroxide is preferably employed in conjunction with hydrogen peroxide to improve pulp brightness.

[0025] The alkali metal hydroxide and carbonate have the effect of swelling wood chips.

[0026] The sulphite is typically employed in a charge of 6 to 25%, by weight, based on the oven dry weight of the wood fibre, whereas the alkali metal hydroxide or carbonate is typically employed in a charge of 0.5 to 10%, preferably, 0.5 to 7%, by weight, based on the oven dry weight of the wood fibre.

[0027] Surprisingly, it was found that the chemical treatment which in turn softens the wood chips permits refining at high refining intensities, which develops the surface characteristics of the wood fibres, collapses the fibre walls, and renders the fibres more flexible with reduced energy consumption but without loss of fibre length and pulp quality, as compared with conventional processes which employ lower refining intensities and consume higher energy.

[0028] It might have been expected that the chemical treatment would render the wood fibres fragile to high intensity refining.

[0029] This new process could be implemented as a single stage followed by subsequent stages of conventional disk refining. It could also be implemented as several consecutive stages to take full advantage of the energy savings over conventional processes. Because of the high stress levels, this approach would also be effective in treating chemimechanical pulp (CMP) screen and cleaner rejects.

EXAMPLES

[0030] The following examples illustrate the nature of the invention.

[0031] Refining Equipment and Procedures:

[0032] The roller mill consisted of four, 160 mm diameter rollers with helical grooves and ridges, rolling inside a cylindrical shell with an inside diameter of 500 mm and a height of 0.9 m. The helical grooves and ridges were both 10 mm wide and the groove depths were 10 mm. For these experiments, the rotational speed of the shaft was 250 RPM and the stress exerted on the material was estimated to be in the range of 300 to 600 kPa with peak strain rates of 200 to 500 s⁻¹.

[0033] The vibratory inertia refiner (VIR) consists of a housing with a spherical support. Mounted on the spherical support is a shaft with a conical rotor; the conical rotor is 155 mm in height with a base of 275 mm and rolls within a conical stator. A debalance weight, installed on bearings within the conical rotor, and is driven independently at 1,000 rpm. The centrifugal force generated by the rotation of the debalancer is transmitted to the conical rotor (by inertia). For experiments with the VIR, the measured stress exerted on the material was between 3 and 30 MPa with strain rates from 50 to 300 s⁻¹, and the average specific energy per impact would be greater than generated by the disc refiner.

[0034] Refiner trials were conducted with an atmospheric 36 inch Bauer Model 400 double rotating disc refiner equipped with conventional Bauer pattern 36104 plates. Variable frequency AC drives permitted each of the machine's motors to operate over a range of 1200 to 1800 RPM. Normal discharge consistency is 25 percent, but lower levels were also explored in the trials described herein.

[0035] The sulphonation in the Examples was carried out with sodium sulphate at pH 9.

Example 1

[0036] Highly sulphonated black spruce chips (sulphonate content ˜2.0% on chips OD basis) were refined in a roller mill and for comparison in an unpressurized refiner operating at rotational speeds of 1200 rpm to produce a chemimechanical pulp (CMP).

[0037] As shown in FIGS. 1 and 2, the specific energy required to produce CMP with a Canadian Standard Freeness of 400 mL and a tensile index of 35 N.m/g with the roller mill was about one third of the disc refining energy.

[0038] At a tensile index of 35 N.m/g this process gives a tear index equal to that of conventional refining (FIG. 3) and a low debris level as measured by means of Somerville shive (FIG. 4). Overall this pulp meets conventional quality specifications but is produced with significantly less energy.

Example 2

[0039] Highly sulphonated black spruce chips (sulphonate content ˜2.0% on chips OD basis) were refined in an unpressurized double-rotating disc refiner at a rotational speed of 1800 rpm. For comparison, these wood chips were refined in the same disc refiner at the conventional disc rotational speed of 1200 rpm.

[0040] As shown in FIG. 5, the specific energy required to produce CMP with a Canadian Standard Freeness of 400 mL with the disc refiner at 1800 rpm was about 33% less than that required at 1200 rpm. The higher intensity provided at the elevated speed permitted a given quality to be obtained at lower specific energy. A tensile index of 35 N.m/g could be reached at 20% lower energy (FIG. 6), with no associated compromise in tear index (FIG. 7).

Example 3

[0041] Highly sulphonated black spruce chips (sulphonate content ˜2.0% on chips OD basis) were refined in an unpressurized double-rotating disc refiner operating at a very high level of intensity by simultaneously rotating the discs at 1800 rpm and lowering the inlet consistency. For comparison, the wood chips were also refined in the novel disc refiner at its conventional intensity.

[0042] As shown in FIG. 8, the specific energy required to produce CMP with a burst index of 2.0 at very high intensity was 52% less than that needed at conventional intensity. It is also evident from FIG. 9 that refining at very high intensity did not compromise tear strength.

Example 4

[0043] Highly sulphonated black spruce chips (sulphonate content ˜2.0% on chips OD basis) were refined in a vibratory inertia refiner (VIR) using either 75 or 100% of the debalancer weight.

[0044] As shown in FIG. 10, the specific energy required to produce CMP with a Canadian Standard Freeness of 400 mL was 33% less with the VIR than the disc refiner. At a tensile index of 55 N.m/g, the VIR required 55% less energy than the disc refiner operating at 1200 rpm (FIG. 11).

Example 5

[0045] Aspen chips treated with alkaline sodium sulphite (10% sodium sulphite charge and 1.4% sodium hydroxide charge on chips, OD basis) were refined in a roller mill and for comparison in a unpressurized refiner operating at rotational speeds of 1200 rpm.

[0046]FIG. 12 shows that the specific energy required to produce aspen CMP with a Canadian Standard Freeness of 200 mL was about 75% less than that required using the disc refiner at 1200 rpm. As seen in FIG. 13, the specific energy required to produce aspen CMP with a tensile index of 50 N.m/g in the roller mill was 70% lower than that used in the disc refiner.

Example 6

[0047] Highly sulphonated black spruce chips (sulphonate content ˜2.0% on chips OD basis) were refined in a roller mill at low specific energy (0.53 GJ/t) followed by an unpressurized disc refiner. These results were compared to experiments on an unpressurized disc refiner operating at rotational speeds of 1200 rpm.

[0048] The specific energy required to produce CMP with a Canadian Standard Freeness of 400 mL using the two stage roller mill/disc refiner process was 40% lower than that required to produce the same pulp with a disc refiner alone, as shown in FIG. 14, The specific energy required to produce CMP with a tensile index of 40 N.m/g, with the roller mill/disc refiner process, shown in FIG. 15, was 25% lower than the specific energy required using the disc refiner alone.

Example 7

[0049] Mixed hardwood chips treated with sodium carbonate were refined using either VIR (2 passes) or the disc refiner (1 pass) to produce a carbonate medium pulp. The refining temperature in the VIR was 20-25° C. FIG. 16 shows that the VIR was significantly more efficient in decreasing the freeness of the pulp than the disc refiner. To produce the medium pulp with a freeness of 300 mL CSF the VIR consumed about 40% less energy than the disc refiner. The concora stiffness of carbonate medium pulp produced in the VIR at a specific energy of about 1 GJ/t was 2-3 times higher than that of pulp produced in the disc refiner, as seen in FIG. 17. 

We claim:
 1. A process for producing a high yield pulp comprising: chemically treating a pulp precursor selected from wood chips or wood fibre to develop wood fibre of said precursor while maintaining fibre integrity, and refining the chemically treated pulp precursor at a high refining intensity.
 2. A process according to claim 1 wherein said refining intensity is in a range of 200 to 2,000 J/kg/impact.
 3. A process according to claim 1 wherein said intensity is in a range of 300 to 700 J/kg/impact.
 4. A process according to claim 1 wherein said high refining intensity is one which establishes a high stress in said chemically treated pulp precursor in a range of 0.1 to 30 MPa at peak strain rates ranging from 50 to 100 s⁻¹.
 5. A process according to claim 1 wherein the chemical treatment comprises sulfonating the precursor to provide a content of SO₃ on wood fibre of 1 to 3%, by weight, based on weight of oven dry wood fibre.
 6. A process according to claim 5 wherein said content is 1 to 2.5% by weight.
 7. A process according to claim 3 wherein the chemical treatment comprises sulfonating the precursor to provide a content of SO₃ on wood fibre of 1 to 2.5%, by weight, based on weight of oven dry wood fibre.
 8. A process according to claim 2 wherein the chemical treatment comprises sulfonating the precursor with charge of alkali metal sulphite at a charge of 6 to 25%, by weight of sulphite, based on the oven dry wood fibre.
 9. A process according to claim 2 wherein the chemical treatment comprises contacting said precursor with a charge of alkali metal carbonate or alkali metal hyroxide at a charge of 0.5 to 10%, by weight, based on weight of oven dry wood fibre.
 10. A process according to claim 2 wherein said precursor comprises wood chips and the chemical treatment softens said wood chips for separation of wood fibres in said wood chips, and develops surface characteristics of the wood fibres of said wood chips, while maintaining fibre integrity.
 11. A process according to claim 2 wherein said precursor comprises wood fibres and the chemical treatment and the refining develop surface characteristics of the wood fibres, while maintaining fibre integrity.
 12. A process according to claim 2 wherein said precursor comprises wood chips and the chemical treatment swells the chips to facilitate separation of wood fibres in the chips, and develops surface characteristics of the wood fibres. 